Multimedia Broadcast and Multicast Services (MBMS) is a broadcasting service offered via cellular communications networks. MBMS is a point-to-multipoint service in which user data is transmitted from a single source entity to multiple recipients. Some exemplary applications of MBMS include file downloads and streaming type services (e.g., mobile television). Enhanced MBMS (eMBMS) is a term used to denote MBMS applications for evolved network systems such as Long Term Evolution (LTE) cellular communications networks.
Data transmission for eMBMS may be accomplished using a MBMS Single Frequency Network (MBSFN). A MBSFN includes multiple network cells all transmitting identical waveforms at the same time. The multiple transmissions are seen by a wireless device on the same network as a single transmission. Accordingly, the wireless device can combine eMBMS transmissions from multiple cells. Transmitting the same data to multiple recipients via eMBMS allows network resources to be shared, thereby improving the efficiency of a wireless communications network in which eMBMS is implemented.
Often, the network cells used in a MBSFN are spread out over multiple base stations. Accordingly, a synchronization process must be performed to ensure that each network cell simultaneously transmits an identical waveform. Generally, such a synchronization process is implemented using an MBMS synchronization protocol (SYNC) administered between a Broadcast Multicast Service Center (BM-SC) and the base stations serving the network cells. The BM-SC acts as an intermediary between a content provider and the base stations in order to facilitate the synchronized transmission of data. According to conventional MBMS SYNC solutions, the BM-SC transmits data in the form of SYNC Protocol Data Units (PDUs). Each SYNC PDU has a corresponding MBMS SYNC time stamp that indicates a time at which the data included in the SYNC PDU is to be transmitted by the base stations in the MBSFN.
One issue with the MBMS SYNC protocol is that the MBMS SYNC timestamps for the SYNC PDUs must account for delays between the BM-SC and the base stations. Conventional solutions fail to provide an accurate way of obtaining the delays between the BM-SC and the base stations in the MBSFN. Although rough estimation can be used to determine the delays, doing so has a large impact on the performance of the cellular communications network. If the estimates of the delays are not long enough, data arrives too late (e.g., too late for transmission of the time indicated by the timestamp) at one of more of the base stations in the MBSFN and is dropped as a result. If the estimates of the delays are too long, an excessive amount of data must be buffered at one or more of the base stations.
One solution proposed in European Patent serial number EP2180651 (A1) describes using a default delay parameter that is long enough to ensure that user data has enough time to reach each base station in the MBSFN before data transmission occurs. However, the use of default delay parameters often results in inefficient over-provisioning, wherein the delay is over-estimated to ensure that user data is not dropped at any of the base stations.
Accordingly, there is a need for an accurate way to determine a delay between the MB-SC and the base stations in a MBSFN to enable synchronized transmission of data in the MBSFN.